1. Field of the Invention
This invention relates to a tibial element for a replacement knee prosthesis of the kind comprising a tibial tray provided with a bearing component having medial and lateral compartments.
2. Description of the Prior Art
Reconstruction of the painful arthritic knee is now a commonplace operation throughout the world with results reported to be of similar success to hip replacements. However, some significant problems relating to knee surgery remain. Among these are wear and damage of articulating surfaces, and repeatability and ease of achieving a stable balanced reconstruction.
In recent years there has been a return to the original concepts of more conforming and constrained knee replacements (i.e., Total Condylar) since concerns as to the long term survivorship of "flat" bearing forms have been raised. Greater congruency between metal and plastic components leads to larger contact areas which are believed to reduce the stresses transmitted to the plastic material. High stress levels are believed to initiate microscopic cracks within the plastic leading to the long term breakdown of the material or wear. Therefore, any reduction in the stress experienced by the plastic is beneficial. However, with increased conformity comes the risk of reduced range of motion from over-constraint and tibial fixation concerns with unloading of the soft tissue structures.
The advent of sliding Meniscal devices sought to combine increased tibiofemoral congruency while restoring normal joint function by allowing the tibial insert to move relative to the base plate.
There are now various meniscal systems available on the market, some with good long-term success, for example as set out in:
Pappas M J, Buechel F F PA1 The New Jersey Low-Contact-Stress Knee Replacement System: PA1 Biomechanical Rationale and Review of the First 123 Cemented Cases. PA1 (Rrch Orthop Trauma Surg 1986; 105: 197-204) PA1 Goodfellow J W, O'Connor J PA1 Clinical Results of the Oxford Knee. PA1 (Clin. Orthop. 1986; 205: 21-42)
However, problems with this type of device have also been reported in the form of insert dislocation and fracture.